The mystery of snowfall

By Ian Roulstone

Updated 7:25 AM ET, Thu February 13, 2014

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Southeast storm moves north – Vehicles are piled up in an wreck Friday, February 14, in Bensalem, Pennsylvania. Traffic accidents involving multiple tractor-trailers and dozens of cars completely blocked one side of the Pennsylvania Turnpike outside Philadelphia.

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Southeast storm moves north – A pedestrian walks through a cloud of steam on a snowy street in New York on February 14. Commuters faced slick roads after a winter storm brought snow and ice to the East Coast.

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Southeast storm moves north – A traveler walks through Ronald Reagan National Airport in Arlington, Virginia, on February 14. Numerous flights were canceled and delayed this week as a result of the snowstorm that pounded a huge section of the country.

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Southeast storm moves north – People dig out vehicles buried in snow in Albany, New York, on February 14.

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Southeast storm moves north – Chris Starace works to clear snow from his roof in Ossining, New York, on Thursday, February 13.

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Southeast storm moves north – Snow and ice cover an Atlanta neighborhood on February 13.

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Southeast storm moves north – People walk through the snow on February 13 in Brooklyn, New York.

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Southeast storm moves north – The lower Manhattan skyline is seen behind a pile of snow in Brooklyn on February 13.

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Southeast storm moves north – After spending the night at Reagan National Airport, Ernie Harmon of Long Island, New York, watches a movie while waiting for his flight February 13.

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Southeast storm moves north – Snow covers cars in Brooklyn on February 13.

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Southeast storm moves north – Claire Lamborne removes snow from her car in Warrenton, Virginia, on February 13.

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Southeast storm moves north – A snowman is seen in front of a hardware store in Washington on February 13.

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Southeast storm moves north – Don Hammond of Newtown, Connecticut, shovels his driveway as snow continues to fall February 13.

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Southeast storm moves north – A long line of travelers winds around the atrium of Hartsfield-Jackson Atlanta International Airport on February 13 as people attempt to catch flights previously canceled because of the massive winter storm.

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Southeast storm moves north – Rescue workers in Oxon Hill, Maryland, attempt to upright an overturned tractor-trailer on Interstate 495 on February 13.

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Southeast storm moves north – Snow collects at the base of escalators at the Dupont Circle Metro Station in Washington on February 13.

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Southeast storm moves north – A postal worker makes his delivery rounds through blizzard conditions in Bethlehem, Pennsylvania, on February 13.

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Southeast storm moves north – Motorists push a vehicle on Highway 70 in Raleigh, North Carolina, on February 13, a day after the worst of the storm struck the town.

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Southeast storm moves north – A man helps push a car in Chapel Hill, North Carolina, on February 13.

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Southeast storm moves north – Snow blankets a street in Ossining, New York, on February 13.

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Southeast storm moves north – A traveler kills time as work crews continue to clear snow from the runways at Reagan National Airport in Arlington, Virginia, on February 13.

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Southeast storm moves north – A man in Bethlehem helps push a stranded motorist February 13.

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Southeast storm moves north – Lyall MacFee shovels snow in front of Sciortino's Pizzeria in Albany, New York, on February 13.

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Southeast storm moves north – A man digs out a small snowplow that got stuck while clearing snow from a sidewalk in Chevy Chase, Maryland, on February 13.

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Southeast storm moves north – A man braces his umbrella while walking through the wind and snow in New York City on February 13.

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Southeast storm moves north – Police work to tow a bus that slid off the road in Philadelphia on February 13.

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Southeast storm moves north – A man walks along the snow-covered Vietnam Veterans Memorial in Washington on February 13.

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Southeast storm moves north – A woman covers her face while walking in New York City on February 13.

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Southeast storm moves north – A worker clears a snowy sidewalk in Washington on February 13.

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Southeast storm moves north – Cars sit covered in snow on a street in Manassas, Virginia, on February 13.

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Southeast storm moves north – A man in Philadelphia shields his face from the elements February 13.

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Southeast storm moves north – A snowplow removes snow from the sidewalk in Washington's Lafayette Park, across the street from the White House, on February 13.

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Southeast storm moves north – People walk through snow February 13 in the Chinatown neighborhood of New York City.

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Southeast storm moves north – People help push a car stuck in the snow February 13 in Alexandria, Virginia.

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Southeast storm moves north – Kevin Miller looks out the passenger window of his friend's car as they sit stuck in traffic during a winter storm in Raleigh on Wednesday, February 12.

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Southeast storm moves north – Traffic moves slowly along Wade Avenue in Raleigh on February 12. Motorists were encouraged to stay off roads.

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Southeast storm moves north – Katharine Newton, a student at the University of North Carolina at Chapel Hill, calls her parents February 12 while walking more than two miles from campus to her parents' home.

Southeast storm moves north – A downed power line is draped across several vehicles in Atlanta on February 12.

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Southeast storm moves north – A police officer redirects traffic in Charlotte, North Carolina, on February 12.

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Southeast storm moves north – Ice coats trees hanging over a sign for the Broadway at the Beach tourist attraction in Myrtle Beach, South Carolina, on February 12.

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Southeast storm moves north – A sign warns drivers of winter weather as they travel on a bleak section of Highway 141 in Norcross, Georgia, on February 12.

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Southeast storm moves north – A truck in Bossier City, Louisiana, blocks access to Interstate 220, which was closed because of icy conditions on February 12.

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Southeast storm moves north – City workers spread a mixture of sand and salt on an intersection in Avondale Estates, Georgia, on February 12.

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Southeast storm moves north – Ice and snow cover Interstate 26 in Columbia, South Carolina, on February 12.

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Southeast storm moves north – Shmetrice Moore, a nurse at an Emory University hospital in Johns Creek, Georgia, scrapes snow and ice off her windshield as she and others are released early from their shift on February 12.

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Southeast storm moves north – Hossam Shalaby waits for his rescheduled flight under a departure board at Hartsfield-Jackson Atlanta International Airport on Tuesday, February 11.

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Southeast storm moves north – A vehicle travels in Greenville, South Carolina, on February 11.

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Southeast storm moves north – People shop for what is left at a Publix grocery store in Decatur, Georgia, on February 11.

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Southeast storm moves north – Vehicles slowly make their way over a snow-covered Route 35 in Fort Payne, Alabama, on February 11.

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Southeast storm moves north – Weather data is projected onto the face of Clint Perkins, director of state operations for the Georgia Emergency Management Agency, as he works in Atlanta on February 11.

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Southeast storm moves north – A vehicle drives through falling snow on the U.S. 421 bypass in Sanford, North Carolina, on February 11.

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Story highlights

Ice and snow storms are crippling some states in the Southeast

Ian Roulstone: Predicting how much snow will actually accumulate is difficult

He says the physics of snowfall is complex, involving factors like temperature, moisture

Roulstone: Making the final call comes down to "seat of the pants" knowledge in the end

A snow-covered landscape is one of the classic images showcasing the beauty of weather on Earth. We are awed by the grandeur of white-capped mountains and the almost magical quality of snow-covered trees. We are also frustrated when the tempests of winter reach far and wide, striking as they have done this year in America's southern states.

When it comes to forecasting the likelihood of a blizzard, the weather anchors know what to say. But when asked to predict how much snow will actually accumulate, they will give estimates. Why?

The computer models used in weather forecasting do not actually predict snow. A single variable is used to predict water in its various forms -- liquid, vapor or ice -- so we need other information, such as air temperature, to decide whether snowfall is likely.

Models forecast the amount of liquid water produced when air rises above the height at which water vapor begins to condense. This is commonly known as the Quantitative Precipitation Forecast (QPF). While QPFs are an important ingredient when it comes to predicting snow, there are many other very subtle factors that can easily tip the balance in favor of rain, or make it very difficult to distinguish between different types of snow.

Ian Roulstone

If temperatures are low enough to allow precipitation to fall as snow, then the forecasters need some way to convert the QPF to an equivalent snowfall. The ratio that we use to calculate the liquid water to snow equivalent is around 1 to 10. That is, if the QPF predicts 1 inch of rain, we can anticipate the amount of snow produced would be 10 inches.

Unfortunately, life's not always that simple.

The liquid water to snow ratio can vary depending on whether the snow is "wet" or "dry." Dry snow is the term used to describe small powdery flakes, and it forms when there is very little moisture available. Under these circumstances, the rain to snow ratio can be considerably higher, with values of 1 to 20 not uncommon.

On the other hand, if there is abundant moisture and the snowflakes are larger and wetter, a ratio of 1 to 5 may be typical. Therefore we need to have a very accurate forecast of the levels of moisture in the atmosphere, together with the variation of temperature with altitude, to even get off on the right foot when it comes to predicting snowfall.

Our forecast models represent a snapshot of the weather at any moment in time by using huge arrays of numbers to describe states of the atmosphere. These numbers represent basic variables such as moisture and temperature.

Calculating how these many millions of "weather pixels" will change requires superfast computation and large amounts of memory. The inevitable limitations on available computer power and data storage force a trade-off between the geographical coverage of models and the detail that we can expect from them. This trade-off can be critical when it comes to calculating reliable QPFs.

Forecasters often have to resort to methods they learned at college, involving dew points, temperature soundings from meteorological balloon ascents and real-time reports from weather stations to assess the impact of a snow storm.

The bottom line is that snow -- one of our most loved, and occasionally loathed, features of weather -- is real tough to forecast well.

We may be able to capture the beauty of Mother Nature with high resolution digital images, but capturing the physics behind the snowfall in our sophisticated weather prediction models is much more challenging.

Making the final call comes down to "seat of the pants" knowledge in the end.